The process for cutting grooves in concrete that has now come to be known as the reflex-percussive process, makes use of repeated, short, sharp hammer blows. The reflex-percussive process may be distinguished from that of the normal pneumatic hammer or drill as follows.
In a common pneumatic drill, a piston reciprocates in a cylinder. Each stroke, the piston strikes an anvil, and on the end of the anvil is a cutting bit. After the blow is struck, the piston retracts. Cutting takes place by a combination of a pounding or crushing action and an abrasive action.
In the conventional pneumatic drill, the anvil may be returned by a spring, or by the exhausted pneumatic air. Either way, the piston retracts as a separate entity from the anvil. The result is that the anvil tends to remain in contact with the surface of the concrete for the fraction of a second required for the shockwave generated by the blow to travel into the concrete, to be reflected back from the undersurface or from the bulk of the concrete, and to re-appear at the surface to "bounce" the anvil clear of the surface.
In reflex-percussion, on the other hand, there is no separately movable anvil. The cutting bit is attached directly to, and is unitary with, the pneumatic piston. In addition, pressurised air acts to withdraw the piston forcibly. Now, the bit is withdrawn from the surface virtually at the instant the blow has been struck, and before the shockwave has had the opportunity to travel into the bulk of the concrete and be reflected back to the surface. The result is that when the reflected shockwave finally does reach the surface, the surface is exposed and unsupported by the mass of the anvil. The material in the surface zone consequently is subjected to a sudden large peak of tensile stress. Concrete is weak in tension, and with many repeated such blows, the surface zone starts to break up.
In reflex percussion, the tool or bit need not be sharply pointed, as is necessary in conventional cutting, to concentrate the energy of the blow. In reflex percussion, the cutting takes place as a result of repeated, relatively light, hammer blows.
The attributes of reflex-percussive cutting may be summarised as follows.
Reflex-percussion produces waste in the form of good-sized chips and pellets. These pellets break away from the surface relatively cleanly, and one of the benefits of reflex-percussion is the reduction in the amount of dust produced during cutting, compared with abrasive or crushing processes such as drilling or sawing.
Reflex-percussion operates efficiently with hard, brittle materials, and materials that are stronger in compression than in tension. Shockwaves are reflected predominantly from the remote surface of the concrete or other material (for example, the undersurface of the road) if that remote surface is well supported. Otherwise, a shockwave can be reflected from the mass or bulk of the material. In contrast to most other cutting processes, the harder the material, the more effective and more efficient the reflex percussion process becomes.
From the standpoint of efficiency, it is important to note that reflex-percussion uses far more compressed air than an ordinary pneumatic drill with the same nominal capacity. This is because air has to be provided under full pressure to return the whole piston/anvil/bit assembly. However, the more appropriate measure of efficiency is the amount of pressurised air required per unit of concrete material removed, and on that scale reflex-percussion is comparatively very efficient.
Machines working on the reflex-percussion principle are shown in, for example, U.S. Pat. No. 3,810,676 (CLARKE, May 1974), U.S. Pat. No. 3,904,245 (CLARKE, September 1975), and U.S. Pat. No. 3,915,582 (CLARKE, October 1975).